In an electrophotographic process or reproduction device such as a copier or a printer, a toner image is formed on the latent electrostatic image of a moving photoconductor. This photoconductor is reusable, and is used to sequentially carry many such toner images. The major portion of each toner image is transferred to the surface of transfer material, as this material and the photoconductor move in close proximity and synchronism through a toner transfer station. The toner image thereafter carried on the surface of the transfer material must now be fused to this surface. In this fusing process the toner image is permanently bound to the transfer material's surface.
Reproduction devices of this type are usually classified as copiers or printers. In a copier the reproduced image is usually provided by scanning an original document's image. In a printer a data processing system, or computer system, usually provides an electronic image that is reproduced into a human readable image.
The present invention will be described relative an electrophotographic printer. However, the scope and spirit of the invention is not to be limited thereto.
A fusing station that has found wide acceptance in the art is the pressure roll fuser. This type of fuser, without limitation thereto, usually includes a pair of circular cylinder rollers that are mounted or supported in generally line contact, to thereby form a fusing nip through which the generally flat transfer material and its toner passes as the toner is fused to the transfer material.
The two rollers of such a roll fuser are conventionally forced or spring biased toward each other so that the transfer material has a force applied thereto as the material passes through the fusing nip. Two types of roll fusers are known in the art, i.e. cold pressure fusers and hot pressure fusers. In a hot pressure fuser the toner being fused is subjected to both heat and pressure. In conventional practice, the fusing nip of such a pressure fuser is maintained closed during passage of the entire length of the transfer material.
Preferred embodiments of the invention include hot pressure fusers, but the invention is not to be limited thereto. Hot pressure fusers may be of the dry release or the wet release type. U.S. Pat. No. 3,912,901, incorporated herein by reference for the purpose of indicating the background of the invention and illustrating the state of the art, is of the wet release type, and also shows a solenoid operated nip opening/closing mechanism.
As electrophotographic reproduction devices such as printers find greater and greater utility, users thereof wish to produce toner images on various types of transfer material, including edge-bound multi-ply transfer material, of which envelopes are a typical example.
Envelopes and other such bound multi-ply transfer material are available in a variety of structural designs and configurations. Variations include envelope construction quality, the type of paper used to form the envelope, the envelope size, the manner in which a single sheet is folded to form the multi-ply envelope, and the paper grain direction of the sheet from which the envelope is formed. In conventional practice, envelopes are manufactured with one surface or panel usually the back panel,of a somewhat larger surface dimension than the opposite panel. In this way, the envelopes interior may expand to form a pocket for holding documents, etc.
We have discovered that roll pressure fusing of multi-ply transfer material, such as envelopes, tends to cause wrinkling of the material by the fusing process. This effect is thought to be caused by the formation of excess material upstream of the fusing nip. Usually, the envelope carries toner to be fused to only one side thereof, and in this case such excess material tends to build up on the non-toner side of the envelope. This excess material moves as a wave toward the envelope's trailing edge (i.e. the last edge of the envelope to pass through the fusing nip). The application of fusing pressure/heat to this excess material can produce an unsightly wrinkled area at the envelope's trailing edge.
We have also noted that standard office practice does not provide or require address or other toner image data to be fused in the region of the trailing edge portion of an envelope.
In accordance with the present invention, an electrophotographic printer fuses a toner image to an envelope by the use of a pressure fuser, and causes the fusing pressure to be released early, i.e. the roll fusing nip opens a predetermined and controlled time/distance before the trailing edge of the envelope exits the fusing nip. As a result, creasing, wrinkling and the like of the envelope is minimized.
Within the knowledge of the inventors hereof, the concept of early fuser roll opening is not known by those skilled in the art.
However, for other purposes, the art teaches early roll closing of a fuser nip. For example, U.S. Pat. No. 4,162,847 discloses a roll fuser wherein the fusing nip is closed before a sheet of transfer material arrives at the fusing nip. This early roll closure is used to cool the hot roll, the hot roll directly engages the relatively cool backup roll during the period of early closure. The effect is to improve performance of the fuser when the transfer material and its toner image subsequently arrives at the fusing nip.
U.S. Pat. No. 4,429,987 is also of this general type having an early roll closure feature.
The problem of fusing envelopes has been recognized in the art. For example, U.S. Pat. No. 4,814,819 attempts to solve the problem of fusing envelopes by providing a heated roller and a pressure roller, each having a resilient layer of critical thermal conductivity, as well as other critical parameters.